So, while it seems to me this is nearly true, I recently found a reseller for Tor books that was also a sort of fansite. They were able to sell me book from the series in both formats (mobi and epub).

My understanding is that Amazon doesn't make the books, they merely sell them (and I believe they get some sort of money out of using the mobi format, beyond just the exclusiving of the format). But if it is the publishers who are releasing the books in mobi, I can think of no reason why they wouldn't be able to sell it in both.